Compositions, processes and methods of improving the wear resistance of prosthetic medical devices

ABSTRACT

Methods and compositions for fabricating prosthetic medical devices exhibiting improved wear resistance include selectively cross-linking polymeric resins then curing and shaping the polymer into a finished article. The selectively cross-linked polymeric compositions may be created by blending a specific amount of cross-linked resins with a specific amount of uncross-linked resins then cured into a polymeric matrix whereby the desired degree or percentage of overall cross-linking is obtained. The polymeric material may then be formed directly into a finished article by injection molding the polymeric material.

FIELD OF THE INVENTION

The present invention relates to polymeric compositions and methods ofmaking the same for use in fabricating prosthetic medical devices, aswell as prosthetic devices made at least partially therefrom withimproved wear resistance.

BACKGROUND OF THE INVENTION

Many prosthetic medical devices are implanted into load-bearing jointssuch as knees, hips, etc. As such, these prosthetic devices must be verystrong and possess a high degree of wear resistance. Presently, theprosthetic medical device industry has utilized various metals andpolymers and combinations thereof to fabricate prosthetic devices.Unfortunately, both metals and polymers have drawbacks. For example,metals such as stainless steel, tungsten and titanium, and alloysthereof, may succumb to the corrosive environment of the body andeventually begin to wear. Such wear may result in fine metallicparticles being scraped away from the contact surface of the device andinto surrounding tissue and bone which may potentially cause pathogenicproblems. Polymers, such as polyethylene, polypropylene and nylons mayalso exhibit wear and may consequently produce particles which diffuseinto tissue and bone. Both metallic and polymeric particles shed fromthese prosthetic medical devices are of concern because they may beinherently reactive with the tissue and bone they contact, thus possiblycausing tissue degradation or necrosis.

Various methods have been devised attempting to reduce the wear rate ofthe load bearing prosthetic medical devices. For polymers, a commonpractice within the prosthetic medical device industry is to usecross-linked polymers and resins to form the medical device. Polymersare commonly cross-linked by chemical catalysis or irradiation exposure.Most cross-linking methodologies do result in greater wear resistance.However, indiscriminate or uncontrolled cross-linking may result in theformation of a weakened polymeric matrix, not capable of withstandingthe enormous pressures placed on the devices in the patient resulting indegradative wear as described above.

Another difficulty conventionally encountered in the manufacturingprocess of prosthetic medical devices is that they cannot be formed byinexpensive injection molding techniques. Instead, these medical devicesmust be formed by extrusion, for example, which requires furthermachining into the finished article. Injection molding,on-the-other-hand, allows for the final article to be formed invirtually one step.

Therefore, a need exists within the prosthetic medical device industryto fabricate an improved polymeric prosthetic device possessingsufficient strength to withstand the stress and pressure imposed on it,yet resist wear so that foreign particles liberated from the prostheticdevice do not cause health problems to the patient. There also exists aneed to fabricate the devices inexpensively by injection molding. Thepresent invention provides compositions, as well as methods of improvingthe wear resistance of prosthetic medical devices, by selectivelycross-linking a polymeric resin using a controlled cross-linking processproviding optimum strength and wear resistance, thus diminishing oreliminating the frequency by which foreign particles are liberated fromthe implanted prosthesis, thereby reducing the risk of compromising thepatient's health. The present invention also provides compositions andmethods of injection molding prosthetic medical devices thus rendering aless expensive, and more facile prosthetic medical device fabricationprocess.

SUMMARY OF THE INVENTION

It has been discovered that by selectively cross-linking components of apolymeric matrix used for prosthetic medical devices, a device can befabricated that possesses the required strength and wear resistance andthereby avoids or reduces the level of polymeric material liberated fromthe device.

One aspect of the present invention provides for a polymeric compositioncontaining mixtures of cross-linked and non-cross-linked polyolefinicresins blended together and ultimately formed into cured polymericarticles. Preferably, the resulting composition and fabricated articlemade from the blended polymeric material of the present inventioncontains cross-linked, linear and branched polyolefinic resins. Thephysical properties of the resulting compositions of the presentinvention have been found to exhibit an unexpectedly high degree of wearresistance and strength. As such, a preferred use of the composition ofthe present invention may be for prosthetic joints or components fordevices for shoulders, elbows, ankles, wrists, fingers, jaws, hips,knees, vertebra, and other load-bearing orthopedic prosthetic medicaldevices. Other preferred prosthetic medical devices fabricated from thecomposition of the present invention include such articles as syringes,catheters and surgical implements requiring a high degree of wearresistance.

In another aspect of the present invention, a method of producing thepolymeric composition is provided. In other aspects of the invention,methods for fabricating prosthetic medical devices made from thepolymeric composition are provided.

A preferred embodiment of the present invention seeks to achieve adesirable balance of wear resistance and high tensile strength andtoughness. A desirable balance is achieved by virtue of combining, in anintegrated matrix, cross-linked and non-cross-linked polyolefinicpolymers and resins. Once blended, the present invention provides forthe mixture to be formed in any suitable manner or otherwise made intoprosthetic medical devices. The finished articles preferably can then beprocessed and packaged for use alone or as components of prostheticmedical devices.

It has been discovered that certain compositions of the presentinvention allow for the blended polymeric mixture to be injectionmolded. This is possible because selected mixture combinations exhibitTheological properties and characteristics which are amenable toinjection molding. Such mixtures exhibit a relatively low viscosityproviding a flowable liquid to be fed into injection molding equipment.

Another aspect of the present invention provides for the use ofpolyolefinic polymers and resins. Within the context of the presentinvention, a polymer is defined as an organic compound having repeatingunits of similar or different monomers. A resin is defined herein as apartially cured polymer having utility as a moldable material suitablefor curing into a solid article. The polymers and resins of the presentinvention have molecular weights ranging from between 1,000 to10,000,000. While the invention preferably uses polyolefinic polymers orresins, any polymer capable of being formed into, and used as,prosthetic devices may be used. Preferably, examples of suchpolyolefinic materials may be polyethylene (PE), polypropylene (PP),high molecular weight polypropylene (HMWPP), high molecular weightpolyethylene (HMWPE), ultra high molecular weight polyethylene (UHMWPE)and ultra high molecular weight polypropylene (UHMWPP), high densitypolyethylene (HDPE), low density polyethylene (LDPE), high densitypolypropylene (HDPP) and low density polypropylene (LDPP). Otherpolymers and resins of the present invention may be polysilanes,polyurethanes, polyethers, polyamides, polyesters, polyalkyl acrylates,nylon, rubber and epoxy resins. It should be understood that the abovelist of polymers is not exhaustive, and other polymers may also beemployed in the present invention.

A further aspect of the invention provides for the use of mixtures ofpolymers and resins, both cross-linked and non-cross-linked varieties,to form a single blended matrix. It is also emphasized that not everypolymer or resin component of the present invention need participate in,or be responsible for, the structural integrity or physicalcharacteristics of the resulting prosthetic medical device, but couldalso serve to improve processing and handling manipulations performed onthe raw materials, intermediate articles and workpieces, as well as thefinished devices.

Another aspect of the invention provides for the use of lubricants,dyes, stabilizers and other processing compounds to be incorporated intothe polymeric matrix. These compounds enhance the polymeric mixture'smanufacturing properties but do not necessarily contribute to thestructural integrity of the final matrix.

In another aspect of the present invention, solid materials may beincorporated into the polymer or resin mixtures. Such solid materialsmay be, for example, chopped carbon or glass fiber or nanotubes, carbonblack, graphite powder, talc, mica, polyamide fiber and other fillerscommonly used in the polymer industry.

In another aspect of the invention, a process is provided wherebypolyolefinic polymers or resins are sealed in a container preferablypurged of most or all oxygen and filled with an inert atmosphere such asnitrogen. Preferably, a powdered form of the polyolefinic polymer resinis irradiated to effect a certain degree of cross-linking to thepolyolefinic polymer or resin. Other cross-linking methods may also beutilized, such as those employed in U.S. Pat. Nos. 5,728,748, 5,650,485,5,449,745, 5,414,049, 5,153,039, 5,160,464, 5,037,928 and U.S.Provisional Application No. 60/130,322, each of which is incorporatedherein as if fully set forth. The irradiated material, now possessing acertain degree of cross-linking, is ready to be blended into a polymericmixture, and processed into a prosthetic device. Optionally, the presentinvention also provides for further irradiation of the finished articleor workpiece. Such subsequent irradiation may be necessary or desirablefor further strengthening or sterilization of the finished article orworkpiece.

In another aspect of the present invention the irradiated polyolefinicpolymer or resin is blended with non-irradiated polyolefinic polymer orresin into a mixture which is then preferably processed and cured intoeither a finished article or unfinished stock article. The processedpolymeric mixture may also be rendered as a powder or pellet, forexample.

DESCRIPTION OF THE FIGURES

FIG. 1 is a representation of data plotting wear rate (mg/mil) vs.percentage of cross-linked polymer incorporated into the final matrix.

FIG. 2 is a representation of data plotting wear rate (mg/mil) vs.percentage of cross-linked polymer incorporated into the final matrix.

FIG. 3 is a representation of a process flow chart diagramming the stepsinvolved in molding the polymeric blends.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

The compositions according to a preferred embodiment of the presentinvention are blends of irradiated or otherwise cross-linkedpolyolefinic polymers or resins combined with linear or uncross-linkedpolyolefinic polymers or resins. These compositions of the presentinvention improve the durability of articles fabricated therefrom byincreasing the wear resistance in load-bearing environments. See FIGS. 1and 2.

The cross-linked, linear and/or branched polyolefinic polymers or resinsmay be the same or different monomeric starting materials. The blendedpolymer or resin mixture may then be processed and cured directly intoprosthetic devices by injection molding, or alternatively into stockarticles or workpieces that may be formed into the desired shape in thefuture. The blended polymer may also be produced in a powder, flake orpellet to be used for future processing.

Generally, the composition of the present invention may be blended tocontain from about 1% to about 99% by weight, based on the total weightof the composition, of a polyolefinic polymer or resin powder which hasbeen irradiated with radiation for a sufficient period of time to causecross-linking of the polyolefinic polymer or resin. In a preferredembodiment, a range of about 1-1,000 Mrads may be used to irradiate thepolyolefin. In an even more preferred embodiment of the presentinvention, 1-100 Mrads may be used, and in the most preferred range 5-20Mrads may be used.

Preferred polymers for irradiation may be selected from one or more ofthe following: polyethylene, polypropylene and/or branched derivativesthereof. In a preferred embodiment, the polymer type is a polyethylene.The polyolefins of the present invention may have a molecular weightranging between about 1000 to about 10,000,000. Preferably, UHMWPE maybe used in the present invention. The ratio of irradiated andnon-irradiated polyolefin may range from about 1-99% irradiatedpolyolefin. In a preferred embodiment, a 50:50 mixture of irradiated andnon-irradiated polyolefinic powder may be blended together. An even morepreferred embodiment of the present invention provides for the mixtureof a 30:70 blend of irradiated to non-irradiated polyolefinic powder.

In yet another preferred embodiment a blend of UHMWPE powder is blendedwith HDPE powder. In this embodiment, either the UHMWPE or the HDPE maybe cross-linked. In an even more preferred embodiment of the presentinvention, the UHMWPE is cross-linked then blended with uncross-linkedHDPE. A preferred ratio blend is 1:99% UHMWPE to HDPE. In a morepreferred embodiment, the ratio may be 20:80 UHMWPE to HDPE. The mostpreferred ratio is 30:70 UHMWPE to HDPE. This mixture is preferablyinjection molded into a prosthetic device.

In another aspect of the invention, a method is provided to prepare theabove described polymeric compositions. In such a method, a selectedpolyolefinic polymer or resin is packaged in an air-tight containerwhich is transparent or opaque. The container is purged of at least mostof the ambient oxygen and, preferably, filled with an inert atmospheresuch as nitrogen or argon. The filled package is then preferablyirradiated using gamma ray, x-ray or electron beam irradiation. Thetotal dose may vary according to the amount of cross-linking desired.After irradiation, the powder is preferably heated to a temperaturebelow, at, or above the melting point of the polymeric material andannealed for a selected period of time at the elevated temperature. Thematerial is then preferably cooled or allowed to cool. The irradiatedmaterial is then mixed with non-irradiated polyolefinic linear polymeror resin. The weight percent for the mixture will vary according to thedesired amount of cross-linking to be contained in the final product asdiscussed above. The mixing may be performed in a blender, rotary mixer,tumbling mill, or any other suitable blending or mixing device. Themixed powder is then extruded or molded into material stock. See FIG. 3.Alternatively, the mixed powder may be injection molded into the desiredshape. If necessary, the final component is then machined into thedesired shape, cleaned and packaged. The packaged article may then besterilized by the use of a non-radiation method such as gas plasma orethylene oxide, or by another irradiation treatment such as those setforth in the above patents and applications which have been incorporatedby reference.

In a preferred embodiment of the composition, UHMWPE is packaged in asubstantially oxygen-free environment. The packaged material is thenirradiated for a total dose of 10-12 Mrads. The package is then annealedat an elevated temperature ranging from approximately 100° C.-150° C.for three days, followed by ambient cooling. The irradiated material isthen blended with non-cross-linked UHMWPE or HDPE to a ratio ofapproximately 5-30% irradiated powder.

See FIG. 2. The powder blend is then preferably melted into a liquidwhich exhibits rheological properties and characteristics suitable forinjection molding applications. The liquid product is then fed intoinjection molding equipment which correspondingly renders the polymerinto a finished article or preformed bar or block. The finished articlemay then be packaged and sterilized using ethylene oxide or gas plasma.The preformed bar or block may be further processed into a finishedarticle.

In another preferred embodiment, additives such as lubricants, dyes,stabilizers and other process enhancing compounds are incorporated intothe polymeric mixture. Such compounds may not necessarily enhance thestrength or structural integrity of the final polymeric matrix, but doaid in the manufacturing process or enhance the overall appearance ofthe finished article. Examples of these compounds may be long chainfatty acids and their salts, organic and inorganic coloring agents, freeradical inhibitors, pH buffering agents and other materials known toenhance processing of polymers within the polymer industry.

In another preferred embodiment of the present invention, solidmaterials may be incorporated into the polymer or resin mixtures. Suchsolid materials may be, for example, chopped carbon or glass fiber ornanotubes, carbon black, graphite powder, talc, mica, polyamide fiberand other fillers commonly used in the polymer industry. As is known inthe polymer industry, such fillers may be advantageously added to apolymer matrix for the purposes of enhancing strength, durability, bulkdensity, machineablity of the resulting polymeric article. Of, coursethe above list is not exhaustive and other uses of the fillers may alsobe contemplated.

In another aspect of the invention the polymeric material is prepared asdiscussed immediately above, then compression molded or extruded into aperformed bar or block. The preformed articles may be shaped intofinished prosthetic medical devices in the future.

EXAMPLE I

GUR1050 (e.g., UHMWPE having average molecular weight of4,000,000-6,000,000) powder was irradiated at 12 Mrads in a nitrogenatmosphere and stabilized in a nitrogen atmosphere at 100° C. for threedays. Several mixtures of the cross-linked irradiated powder wereprepared by blend-mixing with non-irradiated powder the followingproportions

SAMPLE % IRRADIATED, % NON- NUMBER CROSSLINKED IRRADIATED 1  5 95    210% 90% 3 30% 70% 4 100%   0%

A reference sample (Sample 0) containing 0% irradiated and 100%un-irradiated powder was also prepared.

The powder samples were compression molded into blanks which weremachined into cups and then subjected to a hip simulator test todetermine the material.

The results were as follows:

SAMPLE WEAR RATE % REDUCTION % NUMBER (mm³/10⁶ cycles) (from reference)CROSSLINKED 0 (Reference) 102.1 0 0 1 23.1 77.4 5 2 10.6 89.6 10 3 5.894.3 30 4 1.9 98.1 100

The results show that a mixture containing as little as 5% irradiatedmaterial produces an almost 77% reduction in the wear rate. In mixturescontaining 30% irradiated material, the wear reduction is almost 94%.Therefore, mixtures containing from about 5% to about 30% irradiatedmaterial demonstrate an optimum wear reduction versus cost as the costof production of the material increases as the percentage irradiatedmaterial content increases.

EXAMPLE II

GUR1050 (e.g., UHMWPE having average molecular weight of4,000,000-6,000,000) powder was irradiated at 12 Mrads in a nitrogenatmosphere and stabilized in a nitrogen atmosphere at 100° C. for threedays. A mixture of the cross-linked irradiated powder was prepared byblend-mixing with HDPE non-irradiated powder in the followingproportions:

SAMPLE % IRRADIATED, % NON- NUMBER CROSSLINKED IRRADIATED 1 0 100(UHMWPE) 2 30 (UHMWPE) 70 (HDPE)

A reference sample (Sample 1) containing 0% irradiated and 100%non-irradiated powder was also prepared.

The powder samples were injection molded into blanks which were machinedinto cups and then subjected to a hip simulator test to determine thewear rate of the material.

The results were as follows:

SAMPLE WEAR RATE % REDUCTION % NUMBER (mm³/10⁶ cycles) (from reference)CROSSLINKED 1 (Reference) 102.1 0 0 2 4.1 95.9 30

While the foregoing description of examples and figures illustratespreferred embodiments of the various methods, compositions and articlesof manufacture in accordance with the present invention, it should beappreciated that the invention also covers various permutations of theforegoing described features, and that certain modifications may be madein the foregoing without departing from the spirit and scope of thepresent invention which is defined by the claims set forth immediatelyafter.

What is claimed is:
 1. A polymeric composition for fabricatingprosthetic medical devices comprising: a) one or more non-cross-linkedpolyolefinic resin(s) and b) one or more cross-linked polyolefinicresin(s), whereby said one or more non-cross-linked polyolefinicresin(s) and said one or more cross-linked polyolefinic resin(s) areblended and cured into a polymeric article.
 2. The composition of claim1, wherein said non-cross-linked polyolefinic resin(s) are selected fromthe group comprising PE, PP, HMWPE, HMWPP, HDPE, HDPP, LDPE, LDPP,UHMWPE and UHMWPP.
 3. The composition of claim 1, wherein saidcross-linked polyolefinic resin(s) are selected from the groupcomprising PE, PP, HMWPE, HMWPP, LDPE, LDPP, UHMWPE and UHMWPP.
 4. Thecomposition of claim 1, wherein said cross-linked polyolefin resin has amolecular weight between 1,000-10,000,000.
 5. The composition of claim1, wherein said non-cross-linked polyolefin resin has a molecular weightbetween 1,000-10,000,000.
 6. The composition of claim 1, wherein saidcross-linked polyolefin was produced by gamma, electron beam or x-rayirradiation.
 7. The composition of claim 1, wherein said cross-linkedpolyolefinic resin is present in amounts between 1% to 99%.
 8. Thecomposition of claim 1, wherein said cross-linked and non-cross-linkedpolyolefinic resins are blended to homogeneity.
 9. The composition ofclaim 1, wherein said polyolefinic resins are injection molded.
 10. Thecomposition of claim 2, wherein said non-cross-linked polyolefinic resinis UHMWPE.
 11. The composition of claim 3, wherein said cross-linkedpolyolefinic resin is UHMWPE.
 12. The composition of claim 7, whereinsaid cross-linked polyolefinic resin is present in a ratio of 50:50cross-linked to non-cross-linked polyolefinic resin.
 13. The compositionof claim 7, wherein said cross-linked polyolefinic resin is present in aratio of 30:70 cross-linked to non-cross-linked polyolefinic resin. 14.The composition of claim 1, wherein one of said cross-linked andnon-cross-linked polyolefinic resins is UHMWPE and the other of saidcross-linked and non-cross-linked polyolefinic resins is HDPE.
 15. Thecomposition of claim 14, wherein said cross-linked polyolefinic resin isUHMWPE and said non-cross-linked polyolefinic resin is HDPE.
 16. Thecomposition of claim 14, wherein said UHMWPE is present in a ratio of1:99 UHMWPE to HDPE.
 17. The composition of claim 14, wherein saidUHMWPE is present in a ratio of 20:80 UHMWPE to HDPE.
 18. Thecomposition of claim 14, wherein said UHMWPE is present in a ratio of30:70 UHMWPE to HDPE.
 19. The composition of claim 14, wherein saidpolyolefinic resins are injection molded.